MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 435 
_ The biotite appears partly in the exterior portions of the hyper- 
sthene individuals, and partly—and chiefly—grown on to these. 
The hypersthene in several places shows idiomorphic contours 
also against the biotite, the latter must accordingly chiefly have 
been formed at a latter stage than the former. The labradorite 
Fic. 10.—Quartz-norite from Romsaas. (Black=pyrrhotite) 
and the quartz form an intervening mass between the accumula- 
tions of hypersthene, and accordingly crystallized at a somewhat 
later stage. 
The investigation of the Romsaas rocks is in certain respects 
very instructive, but does not fully inform us of the relation of the 
crystallization between the hypersthene and the plagioclase, as 
_ the ferromagnesian silicate of the later stage entered into biotite 
instead of into hypersthene. We are therefore going to investigate 
some rocks where this phenomenon does not appear. | 
